A modular multilevel converter (MMC) is a transformerless power converter suitable for use in high- or medium-voltage power conversion applications. MMCs include multiple switching submodules each having a floating, power-storing capacitor, and an arrangement of one or more switching power transistors. In the control of an MMC, it is necessary to individually voltage-balance and control the floating capacitors in the switching submodules. The switching submodules can be constructed using various configurations of switching power transistors, such as half bridge and full bridge topologies, among others. To control the output of an MMC, the switching power transistors can be controlled using switching control signals generated by an MMC controller.
A significant amount of research has been conducted to mitigate various problems with MMC control, including the reduction of voltage ripple on the capacitors in the switching submodules. Some methods are relatively unsuitable for certain applications, however, for various reasons.